In the conventional art, in electronic devices such as portable telephones and PDAs, thickness reduction and power consumption reduction are advanced. In association with this, further size reduction and further efficiency improvement are desired also in electroacoustic transducers installed in these devices. The most general technique for improving the efficiency in an electroacoustic transducer is to increase the volume of the magnet. Nevertheless, the increase in the volume of the magnet causes an increase in the volume of the electroacoustic transducer itself. Thus, in order to realize size reduction and efficiency improvement, an electrodynamic electroacoustic transducer 200 shown in FIG. 39 is proposed (see, for example, Patent Document 1). Here, FIG. 39 is a structure sectional view of an electrodynamic electroacoustic transducer 200 according to the conventional art.
In FIG. 39, the electrodynamic electroacoustic transducer 200 comprises a first magnet 211, a first yoke 212, a second magnet 213, a second yoke 214, a diaphragm 215, a voice coil 216, and a housing 217.
The first magnet 211 and the second magnet 213 are arrange such as to oppose the both sides of the diaphragm 215 and face each other with the diaphragm 215 in between. A magnetic gap is formed between the first magnet 211 and the second magnet 213. Further, the surfaces opposite to the surfaces opposing the diaphragm 215 in the first magnet 211 and the second magnet 213 are fixed to the first yoke 212 and the second yoke 214, respectively. Furthermore, the first magnet 211 and the second magnet 213 are magnetized such that the polarities should be opposite in the vibrating directions of the diaphragm 215.
The first yoke 212 has a shape such as to surround a surface excluding the surface opposing the diaphragm 215 of the first magnet 211. Similar, the second yoke 214 has a shape such as to surround a surface excluding the surface opposing the diaphragm 215 of the second magnet 213. Further, the first yoke 212 and the second yoke 214 are fixed respectively to the inside of the housing 217.
The diaphragm 215 is fixed to the inside of the housing 217 which has sound holes, and is arranged to be located in surrounded by the first magnet 211, the second magnet 213, and the housing 217. The voice coil 216 is adhered to the diaphragm 215 and is arranged in the magnetic gap. The operation of the electrodynamic electroacoustic transducer 200 is described below.
The first magnet 211 and the second magnet 213 are magnetized in the opposite directions, and arranged in a manner opposing each other. Thus, the magnetic fluxes each emitted from each magnet toward the diaphragm repel each other. Thus, each magnetic flux vector bends almost perpendicularly inside the above-mentioned magnetic gap, and thereby forms a curve directing to the yoke to which each magnet is adhered. Thus, at the position of the voice coil 216 (the voice coil position, hereinafter), a magnetic field is formed that is composed of a magnetic flux perpendicular to the vibrating directions of the diaphragm 215. When a current signal flows through the voice coil 216 arranged on this magnetic flux, a driving force is generated that is proportional to the product between the magnitude of the current and the magnetic flux density at the voice coil position. Then, by virtue of the driving force, the diaphragm 215 vibrates and emits sound.
A general electrodynamic electroacoustic transducer is constructed such that the thickness of the voice coil is thick in the vibrating directions of the diaphragm. In contrast, this conventional art example is constructed such that the thickness of the voice coil 216 is thin in the plane direction of the diaphragm 215. Thus, the overall thickness of the electrodynamic electroacoustic transducer 200 can be made thinner than the conventional art electroacoustic transducer.
Here, in general, in an electrodynamic electroacoustic transducer, when the vibrating part of the diaphragm contacts with a part other than the diaphragm of the transducer, allophone is generated. Thus, design is performed such that even when the maximum sound pressure desired in the transducer is reproduced, the vibrating part of the diaphragm should not contact with any part other than the diaphragm of the transducer. In the structure of the electrodynamic electroacoustic transducer 200 described above, in order that the vibrating part of the diaphragm 215 should not contact with the first magnet 211, the second magnet 213, the first yoke 212, and the second yoke 214 at the time of the maximum amplitude of the diaphragm 215, the distance between each and the diaphragm 215, that is, an amplitude margin, need be ensured sufficiently. Thus, in the structure of the electrodynamic electroacoustic transducer 200 described above, the thickness obtained by adding the thicknesses of the two magnetic circuits (a magnetic circuit constructed from the first magnet 211 and the first yoke 212 and a magnetic circuit constructed from the second magnet 213 and the second yoke 214) and the amplitude margins on the both sides of the diaphragm 215 has been the minimum thickness for the electrodynamic electroacoustic transducer 200.
Further, as an example of an electromagnetic induction type electroacoustic transducer according to the conventional art, an electromagnetic induction type electroacoustic transducer 300 as shown in FIG. 40 is proposed in order to achieve size reduction and efficiency improvement (see, for example, Patent Document 2). Here, FIG. 40 is a structure sectional view of an electromagnetic induction type electroacoustic transducer 300 according to the conventional art.
In FIG. 40, the electromagnetic induction type electroacoustic transducer 300 comprises a magnet 311, a plate 312, a yoke 313, a driving primary coil 314, a diaphragm 315, and a secondary coil 316.
The magnet 311 is fixed on the center axis of the yoke 313 having sound holes. The plate 312 is adhered to the upper face of the magnet 311. The driving primary coil 314 is located on the front face side of the electromagnetic induction type electroacoustic transducer 300 relative to the magnet 311 and the plate 312. Further, the driving primary coil 314, the magnet 311, and the plate 312 are arranged such that the center axes should agree with each other.
The magnet 311 and the driving primary coil 314 are fixed to the yoke 313. The secondary coil 316 is adhered to the diaphragm 315 such as to be located in a magnetic gap formed between the magnet 311 plus the plate 312 and a part of the yoke 313 to which the driving primary coil 314 is fixed. Here, the dimension of the above-mentioned magnetic gap is formed uniformly. The inner periphery of the secondary coil 316 is smaller than the outer periphery of the magnet 311. Further, the outer periphery of the secondary coil 316 is larger than the inner periphery of the driving primary coil 314. Here, the driving primary coil 314 is also fixed to the yoke 313 such as to be located in the above-mentioned magnetic gap. The diaphragm 315 is fixed to the yoke 313 via the edge. The operation of the electromagnetic induction type electroacoustic transducer 300 is described below.
In the electromagnetic induction type electroacoustic transducer 300, when a current flows through the driving primary coil 314, an induction magnetic field is generated that has a magnitude proportional to the time differential of the change of the current. Then, a current is generated in the secondary coil 316 by virtue of the induction magnetic field. In the secondary coil 316, a driving force is generated that is proportional to the product between the current flowing through the secondary coil 316 and the magnetic flux density at the position of the secondary coil 316. By virtue of the driving force, the diaphragm 315 vibrates and thereby emits sound.
In this electromagnetic induction type electroacoustic transducer, in general, the driving primary coil 314 need be arranged in the above-mentioned magnetic gap. This causes an increase in the magnetic gap length by the amount of the driving primary coil 314, and hence reduces the magnetic flux density in the magnetic gap. Thus, a problem arises that the performance is degraded. Thus, in the electromagnetic induction type electroacoustic transducer 300, the magnetic flux is generated in an oblique direction on the front face side relative to the center axis of the diaphragm 315, so that the thickness of the driving primary coil 314 is reduced, so that the magnetic gap length is reduced. As a result, the magnetic flux density at the position of the secondary coil 316 can be increased.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2004-32659
[Patent Document 2] Japanese Laid-Open Patent Publication No. H10-276490